A. Field of the Invention
This invention relates to amplifier protection circuits and more particularly to a fixed ratio VSWR protection circuit which provides power leveling and VSWR protection.
B. Description of the Prior Art
VSWR mismatch protection circuits are used in RF transmitter circuits to insure that power output circuitry is not damaged when the antenna is uncoupled or when the antenna is suddenly damaged or broken. One type of protection circuit employed provides protection by sensing the current to the final amplifier and reducing the current to a preceding stage with increases in final amplifier current. However, it is possible for a transmitter amplifier to be connected to an improper load, such as an open circuit, and still draw the required amount of current. In such a case, the amplifier will dissipate both the power developed, called "forward power", and the power reflected by the improper load, called the "reflected power". If the amplifier is not capable of dissipating the combined forward and reflective power, it can be seriously damaged before any increase in the amplifier current is sensed.
Another type of protection circuit commonly used senses increases in reflected power, and decreases the power of the transmitter in response. Such circuits do not protect against increases in forward power of the transmitter amplifier due to variations in supply voltage and circuit characteristics, which can cause overdissipation of the transmitter amplifier. Some of these prior art circuits continuously cut back the gain of the transmitter amplifier in a linear fashion such that the overall output gain of the amplifier can be less than optimum even though protection is not required. In addition, since the reflected power may increase with an increase in forward power and no allowance is made in the protection circuit for increased forward power, the reflected power sensing mechanism of such prior art circuits can falsely cut back the gain of the transmitter amplifier when the forward power is increased. Thus, it is desirable to provide a protection circuit which controls the output power of a transmitter based upon the ratio of the reflected power to the forward power when that ratio exceeds a certain dangerous threshold.